Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver halide photography (see, for example, “Joho Kiroku (Hard Copy) to Sono Zairyo no Shintenkai (Information Recording (Hard Copy) and New Development of Recording Materials)” published by Toray Research Center Inc., 1993, pp. 241-285; and “Printer Zairyo no Kaihatsu (Development of Printer Materials)” published by CMC Publishing Co., Ltd., 1995, p. 180). Moreover, this system has advantages over silver halide photography: it is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like.
In this dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
As a support of the image-receiving sheet of this type, ordinary paper can be used. In this case, it is possible to produce the image-receiving sheet at low costs. About such an image-receiving sheet using paper as the support, in order to improve the cushion characteristics (cushion properties) of the support, a layer having high cushion characteristics (e.g., a foaming layer made of a resin and a foaming agent) is usually formed between the support and a receptor layer to give more cushion characteristics to the support, thereby improving the contact ability between the image-receiving sheet and a transfer sheet. However, a conventional image-receiving sheet, as described in JP-A-2-265789 (“JP-A” means unexamined published Japanese patent application) or JP-A-8-224967, is produced using an organic-solvent-based resin coating solution. Therefore, the coating solution breaks air bubbles or voids in the foaming layer. The thus-obtained image-receiving sheet cannot achieve desired cushion characteristics, so that white voids and/or density unevenness are generated when an image is formed. Moreover, because of deterioration in heat insulation property of the foaming layer, heat necessary for the transfer of dyes diffuses toward the rear face (back surface) of the image-receiving sheet, thereby causing a problem that the printing sensitivity is lowered.
On the other hand, a method of applying a water-dispersible emulsion onto a support makes it possible to reflect fine unevenness of the foaming layer or the like, onto the surface of a receptor layer. For example, JP-A-2005-62829 describes a receptor layer formed by using a water-dispersible emulsion. The method has advantages that load onto the global environment is small and materials therefor are also relatively inexpensive. However, when using the polymer, there arises a problem that a small amount of water remains in the receptor layer even after drying, and thus the sharpness of images deteriorates.